MOGADISHU, Somalia — The secretary general of the United Nations, António Guterres, arrived in Somalia on Tuesday and appealed for $825 million in aid to address drought and cholera in the East Africa nation on the brink of famine.

Mr. Guterres said the money was needed to help 5.5 million people, about half of Somalia’s population, survive the next six months.

A former prime minister of Portugal who was picked in October to lead the United Nations, Mr. Guterres announced his vi